Audio content enhancement using bandwidth extension techniques

ABSTRACT

Methods, devices and computer program products facilitate the generation of high quality audio content. The audio content includes upper harmonics that are associated with a bass band of frequencies in order to enhance the perception of bass audio components that cannot be produced by the audio speakers. The upper harmonics are generated and processed in such a way that reduces the computational and memory requirements of the audio processing operations. The processed upper harmonics are combined with the original audio that are properly delayed to enhance the quality of the audio content. This Abstract is provided for the sole purpose of complying with the Abstract requirement rules that allow a reader to quickly ascertain the disclosed subject matter. Therefore, it is to be understood that it should not be used to interpret or limit the scope or the meaning of the claims.

FIELD OF INVENTION

The disclosed embodiments generally relate to the field of audioprocessing. More particularly, the disclosed embodiments relate to therestoration of low frequency components of an audio content.

BACKGROUND

This section is intended to provide a background or context to thedisclosed embodiments that are recited in the claims. The descriptionherein may include concepts that could be pursued, but are notnecessarily ones that have been previously conceived or pursued.Therefore, unless otherwise indicated herein, what is described in thissection is not prior art to the description and claims in thisapplication and is not admitted to be prior art by inclusion in thissection.

Accurate reproduction of audio content is an important goal of any audioplayback system. To achieve this goal, various high fidelity audioequipment have been developed to process and subsequently reproduce anaudio content while preserving and, sometimes, enhancing thecharacteristics of the original audio content. These efforts forenhancing the listener's audio experience often requires the use ofexpensive audio processing equipment and multi-channel speaker systems.However, with the increasing popularity of desktop, laptop and portabledevices for accessing and playback of music, movies and othermulti-media content, high-fidelity reproduction of audio content usingrelatively inexpensive speaker systems has become more important.

One of the challenges associated with using inexpensive speakers foraudio playback relates to the reproduction of the bass components of anaudio signal. A bass signal occupies the low-end of the human auditoryrange (i.e., approximately the frequency range between 20-200 Hz) whichoften carries important portions of the overall audio content. Forexample, appreciable portions of a speech signal, and especially thosecorresponding to male voices, are present in the bass range offrequencies, as well as other bass frequencies in signals from musicalinstruments and special effects, such as explosions. Inexpensive audiospeakers, however, cannot reproduce all of the bass components due tophysical limitations. These limitations arise since the speakertransducer dimensions are typically much smaller than the wavelengthsassociated with the audio base components. To overcome theselimitations, high-end speaker systems often utilize sub-woofers that,although more expensive, are physically capable of adequatelyreproducing the bass content.

Alternative methods have also been developed to improve the soundreproduction quality at the lower end of the audible range without theuse of a sub-woofer. One such technique uses a psycho-acousticphenomenon, known as “the missing fundamental,” to create the perceptionof hearing low frequencies by generating audio components in a higherfrequency range. For example, a listener may still perceive a missingfundamental frequency component of say, 100 Hz, if higher harmonics atsay, 200 Hz, 300 Hz, 400 Hz, etc., are present at sufficient amplitudes.As such, a speaker system can create the perception of a bass componentby generating the proper higher order harmonics of the bass content.

SUMMARY

This section is intended to provide a non-exhaustive summary of certainexemplary embodiments and is not intended to limit the scope of theembodiments that are disclosed in this application.

The disclosed embodiments relate to systems, methods, devices, andcomputer program products that enable the production of high fidelityaudio that can be implemented inexpensively. The disclosed embodimentsenable the production of higher harmonics that are combined, with properphase alignments, with the audio content.

One aspect of the disclosed embodiments relates to a method forproducing such an enhanced audio content. Such a method includesperforming a wet chain processing on an input audio content. The wetchain processing includes producing upper harmonics associated with afirst frequency band of the input audio content, where the upperharmonics are located in a second frequency band. The wet chainprocessing also includes filtering the upper harmonics using an infiniteimpulse response bandpass filter to produce a wet chain audio component.The above noted method also includes performing a dry chain processingon the input audio content. The dry chain processing includes generatinga dry chain group delay to match a wet chain group delay associated withthe infinite impulse response bandpass filter, and applying the drychain group delay to the input audio content to produce a dry chainaudio component. The above noted method further provides for combiningthe wet chain audio component with the dry chain audio component.

In one embodiment, the dry chain group delay is generated using at leastone all-pass filter to produce a group delay in the second band offrequencies. Such a group delay matches the group delay associated withthe infinite impulse response bandpass filter of the wet chain. In oneexample, each all-pass filter is a second-order all-pass filter, andeach such all pass filter is configured to exhibit a particular groupdelay characteristic in a sub-band of frequencies within the second bandof frequencies.

In another embodiment, the wet chain processing includes filtering theinput audio content to produce the first band of frequencies. In such anembodiment, the first band of frequencies is produced using a firstinfinite impulse response filter. Further, the dry chain processingincludes generating the dry chain group delay by matching a wet chaingroup delay associated with the first infinite impulse response bandpassfilter of the wet chain.

According to another embodiment, the wet chain processing includesspectral shaping of the wet chain audio component. In particular, thespectral shaping can be carried out using a parametric filter. Forexample, the parametric filter can be configured to emphasize a lowersub-band of frequencies within the second band of frequencies. In stillanother embodiment, the wet chain processing also includes generatingand applying a delay to the wet chain audio component prior to combiningthe wet and dry chain audio components. According to another embodiment,the wet chain processing can include generating and applying a gain towet chain audio component prior to the combining the wet and dry chainaudio components. In yet another embodiment, the dry chain processingincludes generating and applying a gain to the dry chain audio componentprior to the combining the wet and dry audio components.

In one embodiment, the input audio content is a single-channel audiocontent, and the dry chain processing and wet chain processing arecarried out on the single-channel audio content. In another embodiment,the input audio content is a multi-channel audio content, and the drychain processing and wet chain processing are carried out on individualchannels of the multi-channel audio content. In still anotherembodiment, the input audio content is a multi-channel audio content,the dry chain processing is carried out on individual channels of themulti-channel audio content; and at least a portion of the wet chainprocessing is carried out on a combined audio content that comprises twoor more of the channels of the multi-channel audio content.

Another aspect of the disclosed embodiments relates to a device thatincludes a processor and a memory that includes processor executablecode. The processor executable code, when executed by the processor,configures the device to perform a wet chain processing on an inputaudio content by configuring the device to produce upper harmonicsassociated with a first frequency band of the input audio content (wherethe upper harmonics being located in a second frequency band), and tofilter the upper harmonics using an infinite impulse response bandpassfilter to produce a wet chain audio component. The processor executablecode, when executed by the processor, also configures the device toperform a dry chain processing on the input audio content by configuringthe device to generate a dry chain group delay to match a wet chaingroup delay associated with the infinite impulse response bandpassfilter, and to apply the dry chain group delay to the input audiocontent to produce a dry chain audio component. The processor executablecode, when executed by the processor, further configures the device tocombine the wet chain audio component with the dry chain audiocomponent.

Another aspect of the disclosed embodiments relates to a computerprogram product that is embodied on a non-transitory computer readablemedium. The computer program product includes computer code forperforming a wet chain processing on an input audio content. The wetchain processing includes producing upper harmonics associated with afirst frequency band of the input audio content (where the upperharmonics are located in a second frequency band), and filtering theupper harmonics using an infinite impulse response bandpass filter toproduce a wet chain audio component. The computer program product alsoincludes computer code for performing a dry chain processing on theinput audio content. The dry chain processing includes generating a drychain group delay to match a wet chain group delay associated with theinfinite impulse response bandpass filter, and applying the dry chaingroup delay to the input audio content to produce a dry chain audiocomponent. The computer program product further includes computer codefor combining the wet chain audio component with the dry chain audiocomponent.

These and other advantages and features of the disclosed embodiments,together with the organization and manner of operation thereof, willbecome apparent from the following detailed description when taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosed embodiments are described by referring to the attacheddrawings, in which:

FIG. 1 illustrates a system that is used to generate an enhanced audiocontent in accordance with an example embodiment;

FIG. 2 illustrates another system that is used to generate an enhancedaudio content in accordance with an example embodiment;

FIG. 3 is a plot of an audio content containing higher order harmonicsthat is generated in accordance with an example embodiment;

FIG. 4 illustrates the frequency response of a harmonic shaping inaccordance with an example embodiment;

FIG. 5 illustrates a procedure for designing and selecting variouscomponents for processing an input audio content in accordance with anexample embodiment;

FIG. 6 illustrates magnitude plots associated with a finite impulseresponse (FIR) and an infinite impulse response (IIR) that are producedin accordance with an example embodiment;

FIG. 7 illustrates a group delay associated with an IIR filter that isproduced in accordance with an example embodiment;

FIG. 8 illustrates group delay plots associated with the first stage ofa delay compensation design procedure in accordance with an exampleembodiment;

FIG. 9 illustrates group delay plots associated with the second stage ofa delay compensation design procedure in accordance with an exampleembodiment;

FIG. 10 illustrates group delay plots associated with the third stage ofa delay compensation design procedure in accordance with an exampleembodiment;

FIG. 11 illustrates group delay plots associated with the fourth stageof a delay compensation design procedure in accordance with an exampleembodiment;

FIG. 12 is a block diagram of a procedure for producing an audio contentcontaining higher order in accordance with an example embodiment; and

FIG. 13 illustrates a device within which the disclosed embodiments maybe implemented.

DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS

In the following description, for purposes of explanation and notlimitation, details and descriptions are set forth in order to provide athorough understanding of the disclosed. However, it will be apparent tothose skilled in the art that the present invention may be practiced inother embodiments that depart from these details and descriptions.

Additionally, in the subject description, the word “exemplary” is usedto mean serving as an example, instance, or illustration. Any embodimentor design described herein as “exemplary” is not necessarily to beconstrued as preferred or advantageous over other embodiments ordesigns. Rather, use of the word exemplary is intended to presentconcepts in a concrete manner.

As noted earlier, higher order harmonic components can be generated tomimic the presence of lower frequencies that cannot be adequatelyproduced by inexpensive audio equipment. Such higher order components,once generated, are combined with the original audio content. It isimportant, however, to ensure that the higher order harmonics have theproper magnitude and are combined with the original audio with theproper time alignment. It is also important to generate such harmonicswith the proper magnitude and phase characteristics while minimizing thecomputational complexity of the audio processing operations. Thereduction of computational cycles and memory usage can be significantfactors in implementations within, for example, mobile devices that havea limited memory, processing capability and battery life.

The disclosed embodiments enable the generation of an enhanced audiooutput by generation higher order harmonics that are properly combinedwith an input audio content while minimizing the usage of computationalresources. FIG. 1 illustrates some of the components that are used togenerate such an enhanced audio content according to an exemplaryembodiment. The specific configuration of the audio processing apparatusthat is depicted in FIG. 1 relates to the generation of an enhancedaudio output for a stereo input content comprising a left-channel audioinput 102A and a right-channel audio input 102B. Each of the left andright audio channels are processed by the various components orprocesses labeled 106 to 116 in the wet-channel chain 104. Inparticular, the left-channel audio input 102A is first processed by thefirst bandpass filter 106A, which isolates the frequencies in oneparticular range, i.e., band 1. By the way of example, and not bylimitation, band 1 may comprise frequencies in the range 80-150 Hz. Inone embodiment, the first bandpass filter 106A is an Finite ImputeResponse (FIR) filter. In another embodiment, the first bandpass filter106A is a quasi-linear phase Infinite Impulse Filter (IIR). Furtherdetails regarding the first bandpass filter 106A will be described inthe sections that follow. The characteristics of FIR and IIR filters, aswell as the advantages of one over the other, are well known in the areaof signal processing and will not be elaborated in detail. However, itis important to note that while FIR filters are inherently stable andcan be readily designed to produce a linear phase, they often requireconsiderably more computational cycles and memory usage than IIR filterswith similar filtering characteristics.

Referring back to FIG. 1, the output of the first bandpass filter 106Ais processed by the non-linear processor 108A. The non-linear processor108A is configured to produce higher harmonics associated with thefrequency content of band 1. By the way of example, and not bylimitation, the non-linear processor 108A may be configured to generatethe higher order harmonics, even and odd, by using a smoother withadjustable rising and falling time constants. The output of the smootheris based on both the present input signal and a previously smoothedoutput signal. The proportion between the two input signals can dependon whether the signal is increasing or decreasing and on the timeconstants. In another embodiment, the non-linear processor 108A can usehalf- and full-wave rectification procedures, while in other embodimentsintegration and clipping methods can be used to produce the higher orderharmonics. FIG. 3 is an exemplary plot of the normalized output of thenon-linear processor (NLP) 108A that is produced from an 80 Hz inputsinewave. As evident from the exemplary plot of FIG. 3, the non-linearprocessor (NLP) 108A has produced higher order harmonics (e.g., at 160Hz, 240 Hz, etc.)

Returning to FIG. 1, the second bandpass filter 110A receives the inputfrom the non-linear processor 108A that comprises both the frequencycomponents in band 1 and the associated higher order harmonics. Thesecond bandpass filter 110A, next, processes its input to isolate thehigher order harmonics that reside in band 2. By the way of example, andnot by limitation, band 2 may comprise frequencies in the range 150-300Hz. In one embodiment, the second bandpass filter 110A is an FIR filter.In another embodiment, the second bandpass filter 110A is a quasi-linearphase IIR filter.

The output of the second bandpass filter 110A is next processed by thespectral shaper 112A. The spectral shaper 112A is configured to shapethe spectrum of the higher harmonics in band 2. In one example, thespectral shaper 112A serves to level out the lower frequency range ofharmonics that are passed through the second bandpass filter 110A. Inone example, the spectral shaper 112A implements an IIR spectral shapingfilter. Depending on the order of the bandpass filter 110A, there existsa certain degree of rolloff within the passband of the second bandpassfilter 110A approaching the cutoff frequencies. In one embodiment, thespectral shaper 112A of FIG. 1 is configured to implement a spectralshaping filter that is centered near the lower cutoff frequency of thesecond passband filter 110A. In this configuration, the shaping filteremphasizes the lower frequency components of the wet chain prior to thesummation with the dry chain signals. The spectral shaping filter thatis implemented by the spectral shaper 112A can be a parametric filterand centered with respect band 2. Further, the Q factor and the gainassociated with the spectral shaping filter can be tuned to produce thedesired aural effects. For example, the tuning of the spectral shapingfilter can be carried out through listening tests. FIG. 4 is anexemplary plot of the spectral shaper frequency response that may beapplied to the band 2 harmonics of 150-300 Hz frequency range. In theexemplary plot of FIG. 4, the spectral shaper center frequency is 150Hz, with a sharp rolloff on either side of this center frequency.

FIG. 1 further illustrates that the output of the spectral shaper isprocessed by the delay generator 114A. The delay generator 114A producesthe appropriate offset delay that may become necessary for the propercombination of the wet and dry chain components. In embodiments thatutilize FIR bandpass filters, the delay generator 114A may be omitted orbe configured to not to modify its input signal. Due to the linear phasecharacteristic of the FIR filters, all frequencies are subject to thesame delay (i.e., the same number of samples) and, therefore, the drychain delay compensator 118 may adequately compensate the delaysassociated with the wet chain 104. However, in embodiments that utilizeIIR bandpass filters, the delay generator 114A can produces the requireddelay to compensate, at least partially, for the group delay associatedwith the IIR filters. Referring back to FIG. 1, the gain generator 116Aproduces the appropriate gain and applies it to the audio content thatis input to the gain generator 116A. In one example, the gain generator116A is configured as a constant gain multiplier. The gain value that isapplied by the gain generator 116A can be tuned to achieve a desiredlevel of harmonic content at its output and to prevent unwanted audioartifacts. For example, the gain generator 116A can apply gain values toavoid clipping of the output audio. The output of the gain generator116A thus comprises spectrally shaped band 2 components that areappropriately scaled and comprise the proper offset delay (if needed).

Referring back to FIG. 1, the processing of the right-channel inputaudio 102B in the wet chain 104 can be conducted using similar audioprocessing components and similar audio processing procedures that weredescribed in connection with the left-channel audio input 102A. Inparticular, the right-channel input audio 102B is processed by the firstbandpass filter 106B to isolate the frequency components in band 1. Band1 of the right-channel may span the same or different range offrequencies as band 1 of the left-channel. The output of the firstbandpass filter 106B is then processed by the non-linear processor 108B,which produces higher order harmonics. A second bandpass filter 110Bnext isolates the higher order harmonics that reside in band 2 of theright-channel audio. Band 2 that is associated with the right-channelmay span the same or different range of frequencies as band 2 of theleft-channel. Spectral shaper 112B, delay generator 114B and gaingenerator 116B in the right-channel audio path process the audio contentsimilar to the processes discussed in connection with the left-channelcomponents.

Still referring to FIG. 1, in parallel with the wet channel processing,the input audio content is processed in the dry chain 128. Inparticular, the left-channel input audio 102A is processed by the groupdelay compensator 118. The group delay compensator 118 is configured tomatch the group delay associated with the various filters ofleft-channel audio in the wet chain 104. Further details regarding thevarious operations and components of the group delay compensator 118will be discussed in the sections that follow. The output of the groupdelay compensator 118 that corresponds to the left audio channel is nextscaled by the appropriate gain values in the gain generator 120A.Similarly, the right-channel audio input 102B is processed by the groupdelay compensator 118 to match the appropriate group delay associatedwith the wet-channel processing for this input channel. The output ofthe group delay compensator that is associated with the right channel isthen scaled properly in the gain generator 120B. In one embodiment, thegain generator 120A and 120B apply suitable gain values to the dry chainaudio channels to avoid clipping of the output audio (i.e., oncecombined with the wet channel components). In one example embodiment,the gain value of one or both gain generators 120A and 120B is set tounity. It should be noted that while a single block is used to depictthe group delay compensator 118, this block may comprise separatecomponents or blocks that are configured for processing the left- andthe right-channel input audio signals, respectively.

Finally, the left-channel audio output 124A and the right-channel audiooutput 124B are generated by combining the outputs of the dry chain gaingenerators 120A and 120B with the outputs of the wet chain gaingenerators 116A and 116B, respectively. For example, the combiners 122Aand 122B can be configured to add the dry and wet channel components.The output audio that is generated after the exemplary processing stagesof FIG. 1 contains higher order harmonics that are associated with bassfrequency contents of the input audio. These higher order harmonics areproduced by the wet chain and are appropriately shaped, delayed andscaled to eliminate or reduce unwanted audio artifacts. The dry chainphase matching that is carried out by the group delay compensator 118further compensates for the group of the wet chain IIR filters, therebyenabling seamless combination of the dry chain and wet chain audiochannels to produce a high quality audio output. Such an output audioprovides an enhanced listening experience that includes the perceptionof a rich bass signal even when with inexpensive speaker systems, suchas the ones used in television sets and portable multimedia players.

It should be noted that in order to facilitate the understanding of theunderlying concepts associated with the disclosed embodiments, theprocessing components or stages the wet and dry chains of FIG. 1 aredepicted as separate blocks that operate on separate audio channels.However, it is understood that one of more of the depicted componentsmay be combined with other components. One exemplary combinationincludes utilizing a single filter block to carry out bandpass filteringof the left and right channels. Alternatively, or additionally, thevarious blocks within the same or different processing chain(s) maycomprise common components that are shared between those blocks. Thedisclosed system can also comprise additional components or blocks (notdepicted in FIG. 1) that are used for carrying out common audioprocessing operations including, but not limited to, analog-to-digital(A/D) and digital-to-analog (D/A) conversions, normalization,equalization, resampling and the like. Further, while FIG. 1 illustratesthe processing of stereo channels, it is understood that additionalaudio channels can be similarly processed. Moreover, it should beunderstood that additional connections between the various component ofFIG. 1 may exists that have not been explicitly depicted for the sake ofsimplicity. For example, the component that are in the dry chain 128 canbe in communication with at least some of the components that arelocated in the wet chain 104.

FIG. 2 illustrates an alternate embodiment of the present invention, inwhich the left-channel input audio 202A and the right-channel inputaudio 202B are first combined (e.g., added) by the combiner 204 and thenprocessed by the first bandpass filter 206, the non-linear processor208, the second bandpass filter 210, the spectral shaper 212, the delaygenerator 214 and the gain generator 216. These processing blocksperform similar operations as the ones discussed in connection withFIG. 1. Similar to FIG. 1, the dry chain processing is carried bymatching the delays associated with the various stages of the wet chainin the group delay compensator 220. However, unlike the phase matchingoperation of FIG. 1, the group delay compensator only needs to accountfor the delays associated with the single path processing blocks of thewet chain 226. As such, the same group delay compensation is providedfor both the left-channel input audio 202A and the right-channel inputaudio 202B. The gain generators 220A and 220B subject the phase-matchedleft and right audio channels to the appropriate scaling factors,respectively. Finally, the left-channel audio output 224A and theright-channel audio output 224B are generated by combining the output ofthe wet chain gain generator 216A to the outputs of the dry chain gaingenerators 220A and 220B. It is evident that, due to the singleprocessing path of the wet chain 226, the exemplary embodiment that isdepicted in FIG. 2 has a lower computational complexity than the onedescribed in FIG. 1. However, the system of FIG. 1 may provide superioroutput audio quality, especially if the audio content of the left andthe right input channels are sufficiently different from one another.

As noted above in connection with FIG. 1, FIR filters typically requiremore computational cycles and/or memory resources compared to their IIRcounterparts. Therefore, the processing requirements for implementingFIR filters may render the implementation of such filters infeasible incertain applications. On the other hand, IIR filters that are used inaudio processing applications may produce poor audio quality due totheir non-linear phase characteristics. According to the disclosedembodiments, quasi-linear-phase low-order IIR filters can be used inplace of the FIR filters. The multiple quasi-linear-phase low-order IIRfilters consume significantly lower computational resources compared tothe longer duration linear-phase FIR filters, while producing thedesired linear (or quasi-linear) phase characteristic for thefrequencies of interest. The audio signals that are processed using thedisclosed quasi-linear-phase low-order IIR filters exhibit substantiallyimproved objective and subjective audio quality when compared to theaudio signals that are processed using arbitrary group-delay IIRfilters. In some embodiments, quasi-linear-phase low-order IIR filtersare designed and selected using the BU-method (i.e., a design procedureoriginally introduced by Harmut Brandenstein and Rolf Unbehauen in anarticle titled “Least-Squares Approximation of FIR by IIR DigitalFilters,” published in IEEE Transactions on Signal Processing, Vol. 46,No. 1, in January 1998). The IIR filters that are selected according tothe BU-method can be selected to reasonably match the amplitude responseof the corresponding FIR filter. Such IIR filters also produce aquasi-linear phase in the passband. However, the phase characteristicsof the quasi-linear IIR filters still exhibit non-linearities in (e.g.,deviations from an average value or a straight line phasecharacteristic) the passband of interest. Therefore, a simple integerdelay in the dry chain cannot adequately align the dry chain and wetchain audio components. According to the disclosed embodiments, thegroup delay associated with the IIR filters in the wet chain arecompensated in the wet chain using to enable the proper alignment of thewet and dry audio chains.

FIG. 5 illustrate a procedure for the selection of the various filtersand compensating the associated group delays in accordance with anexample embodiment. At 502, the cutoff frequency, f_(C), (e.g., the 3 dBfrequency) point in speaker system is determined. The cutoff identifiesan upper range of frequencies for the bass range of interest. The valueof f_(C) is used in subsequent operations to select a bandpass filterwith proper characteristics. In one example, the value of f_(C) is 150Hz. At 504, an IIR filter for band 1 frequencies is selected. Forexample, band 1 frequencies may span 80-150 Hz. The filter at 504 can bedesigned and selected according to the BU-method and can include a quasilinear-phase IIR filter. Such a filter may also has a low decay rolloff(e.g., 10 dB per octave) for the selected band 1 frequencies. The IIRquasi linear-phase IIR filter at 504 is designed and selected to matchthe corresponding FIR filter's magnitude response. FIG. 6 shows themagnitude frequency response of the IIR and the FIR filters that aredesigned for the band 80-150 Hz in accordance with an exemplaryembodiment.

Referring back to FIG. 5, at 506, the group delay associated with theband 1 IIR filter is determined. The concept of group delay can bebetter understood by considering the following. The output, y(t), of alinear, time-invariant system (such as the FIR's and IIR's that aredescribed in the disclosed embodiments), which is characterized by atransfer function H(iw), for a complex sinusoid input, x(t)=e^(iwt), isgiven by:

$\begin{matrix}\begin{matrix}{{y(t)} = {{H\left( {\; w} \right)}^{\; {wt}}}} \\{= {\left( {{{H\left( {\; w} \right)}}^{{\varphi}{(w)}}} \right)^{\; {wt}}}} \\{= {{{H\left( {\; w} \right)}}{^{{({{wt} + {\varphi {(w)}}})}}.}}}\end{matrix} & (1)\end{matrix}$

The phase shift, φ(w), introduced by such a system, is defined as:

φ(w)=arg{H(iw)}  (2).

The group delay, τ_(g), is determined by taking the negative of thefirst derivative of this phase shift:

$\begin{matrix}{\tau_{g} = {- {\frac{{\varphi (w)}}{w}.}}} & (3)\end{matrix}$

The group delay associated with an exemplary IIR filter that is designedand selected for the band 80-159 Hz is depicted in FIG. 7.

At 508 in FIG. 5, a dry chain group delay is generated to match the band1 group delay associated with the band 1 IIR filter. In one embodiment,a plurality of all-pass filters are used to produce the desired groupdelay in the dry chain. In particular, the dry chain group delaycompensation can be designed in stages, where each stage uses a singlesecond-order all-pass filter. Each all-pass filter, by definition,passes all frequencies equally in terms of magnitude. However, theall-pass filters can be tuned to produce a phase shift of varyingdegrees in certain range of frequencies. In one embodiment, a pluralityof all-pass filters are configured in a cascade fashion, such that theoutput of a first all-pass filter is fed into the input of a secondall-pass filter, the output of the second all-pass filter is fed intothe input of a third all-pass filter and so on. In this fashion, eachall-pass filter can be configured to control the phase of the audiocontent within a particular sub-region of the frequency band, therebyproducing an aggregate group delay that matches the wet chain groupdelay over the frequency band of interest.

FIGS. 8 through 11 illustrate the different stages of the dry chaindelay generation for the band 80-150 Hz in accordance with an exemplaryembodiment. In particular, FIG. 8 illustrates stage 1 of the designprocess where the group delay for wet chain IIR filter (i.e., also shownin FIG. 7) is plotted along with a group delay associated with a singlesecond order all-pass filter. FIG. 9 illustrates the same two plots thatare depicted in FIG. 8, in addition to the plot associated with asecond-order all-pass filter that is used in stage 2 of the designprocess. FIG. 9 also depicts an aggregate group delay plot (i.e.,labeled as “summed compensation) that is produced by adding the groupdelay plots of stage 1 and stage 2 all-pass filters. Examination of FIG.9 reveals that the shape of the summed compensation plot is starting toresemble the IIR group delay. FIG. 10 shows stage 3 of the design, wherean additional second-order all-pass filter is added. The “summedcompensation” plot of FIG. 10 is produced by adding the group delayplots associated with all three all-pass filters. As is evident fromFIG. 10, the summed compensation plot closely resembles the IIR groupdelay plot except for a constant delay value (i.e., a vertical shift)across all frequencies. FIG. 11 illustrates that the IIR filter groupdelay may be shifted to match the dry chain delay. This operation willbe further discussed in connection with operation 520 of FIG. 5.

Referring back to FIG. 5, after the generation of the appropriate delayat 508, an IIR filter for band 2 is selected at 510. The selection ofthe IIR filter at 510 can be carried out similar to the operationdiscussed in connection with 504 for band 2 frequencies (e.g., 150-300Hz). At 512, the group delay associated with band 2 IIR filter isdetermined and, at 514, the appropriate dry chain group delay isdetermined to match band 2 IIR filter group delay. Operations at 512 and514 can be conducted similar to those discussed in connection withoperations 506 and 508. At 516, parametric shaping is performed. Asdiscussed in connection with the spectral shaper 112A in FIG. 1, thespectral shaping can be carried out to emphasizes the lower frequencycomponents of the wet chain prior to the combination with the dry chaincomponents. In one configuration, the parametric filter is centeredaround the lower cutoff frequency of band 2. The Q factor, which is theratio of the filter's center frequency to its bandwidth, as well as thegain of the filter can be evaluated and tuned through listening tests.

At 518, the zeros of the parametric filter that was designed inoperation 516 are transformed inside the unit circle to make the filtera minimum-group delay filter. A minimum group delay (and more generallya minimum-phase filter) has all of its poles and zeros within the unitcircle and, therefore, is both stable and causal. At 520, the linearphase delay is determined for the wet and/or dry chains. As notedearlier in connection with FIG. 11, the aggregate group delay associatedwith the all-pass filters (e.g., in operations 508 and 514) can beoffset from the desired wet chain group delay. In operation 520, thenecessary linear delay is calculated to match the group delays of thetwo chains. The exemplary plots of FIG. 10 and FIG. 11 illustrate thatthe IIR group delay in the wet chain can be offset by a linear delay(e.g., a fixed number of samples) to align the group delays of the wetand dry chains.

It should be noted that the exemplary block diagram of FIG. 5illustrates the matching of the group delays for each IIR filter that isconducted in separate operations 508 and 514. However, it is understoodthat the matching can be conducted based on the overall output of thewet chain. Further, the matching of the wet chain group delay wasillustrated in the exemplary FIGS. 8-10 to comprise three stages. It isnoted, however, that fewer or additional stages can be used to match thedesired group delay characteristics.

FIG. 12 illustrates the operations that are conducted to produce aprocessed audio content containing higher order harmonics in accordancewith an exemplary embodiment. The wet chain operations are depicted asoperations 1202 through 1212, whereas the dry chain operations aredepicted as operations 1214 through 1216. At 1202, band 1 frequenciesare produced by bandpass filtering a portion of the input audio content.At 1204, the upper harmonics associated with band 1 are generated. At1206, the upper harmonics are bandpass filtered to isolate the harmonicsthat reside within band 2 of frequencies. At 1208, the filtered upperharmonics are spectrally shaped. For example, a parametric filter can beused to preferentially emaphasize a sub-band of frequencies (e.g., alower sub-band of frequencies) within the band 2 frequency range. At1210, the wet channel delay, if needed, is generated. At 1212, anappropriate gain is determined and applied to the wet chain audiocomponents.

At 1214, as part of the dry chain processing, the appropriate dry chaindelay is generated and applied to the input audio content. As describedearlier, such a delay is intended to match the group delay associatedwith one or more of the IIR filters of the wet chain. At 1216, anappropriate gain is determined and applied to the dry chain audiocomponents. In some embodiments, the application of only one of the wetchannel gain (determined at 1212) or the dry channel gain (determined at1216) may be sufficient for producing a properly scaled audio content.As such, one of the operations at 1212 or 1216 may be omitted. Finally,at 1220, the dry chain audio components and the wet chain audiocomponents are combined to produce the final audio content.

It is understood that the disclosed embodiments may be implementedindividually, or collectively, in devices comprised of various hardwareand/or software modules and components. These devices, for example, maycomprise a processor, a memory unit, an interface that arecommunicatively connected to each other, and may range from desktopand/or laptop computers, to consumer electronic devices such as mediaplayers, mobile devices and the like. For example, FIG. 13 illustrates ablock diagram of a device 1300 within which the various disclosedembodiments may be implemented. The device 1300 comprises at least oneprocessor 1302 and/or controller, at least one memory 1304 unit that isin communication with the processor 1302, and at least one communicationunit 1306 that enables the exchange of data and information, directly orindirectly, through the communication link 1308 with other entities,devices and networks. The communication unit 1306 may provide wiredand/or wireless communication capabilities in accordance with one ormore communication protocols, and therefore it may comprise the propertransmitter/receiver antennas, circuitry and ports, as well as theencoding/decoding capabilities that may be necessary for propertransmission and/or reception of data and other information. Theexemplary device 1300 that is depicted in FIG. 13 may incorporate someor all of the components that are depicted in FIGS. 1 and 2.

Similarly, the various components or sub-components within each modulethat is depicted in FIGS. 1 and 2 may be implemented in software,hardware or firmware. The connectivity between the modules and/orcomponents within the modules may be provided using any one of theconnectivity methods and media that is known in the art, including, butnot limited to, communications over the Internet, wired, or wirelessnetworks using the appropriate protocols.

Various embodiments described herein are described in the generalcontext of methods or processes, which may be implemented in oneembodiment by a computer program product, embodied in acomputer-readable medium, including computer-executable instructions,such as program code, executed by computers in networked environments. Acomputer-readable medium may include removable and non-removable storagedevices including, but not limited to, Read Only Memory (ROM), RandomAccess Memory (RAM), compact discs (CDs), digital versatile discs (DVD),etc. Therefore, the computer-readable media that is described in thepresent application comprise non-transitory storage media. Generally,program modules may include routines, programs, objects, components,data structures, etc. that perform particular tasks or implementparticular abstract data types. Computer-executable instructions,associated data structures, and program modules represent examples ofprogram code for executing steps of the methods disclosed herein. Theparticular sequence of such executable instructions or associated datastructures represents examples of corresponding acts for implementingthe functions described in such steps or processes.

The foregoing description of embodiments has been presented for purposesof illustration and description. The foregoing description is notintended to be exhaustive or to limit embodiments of the presentinvention to the precise form disclosed, and modifications andvariations are possible in light of the above teachings or may beacquired from practice of various embodiments. The embodiments discussedherein were chosen and described in order to explain the principles andthe nature of various embodiments and its practical application toenable one skilled in the art to utilize the present invention invarious embodiments and with various modifications as are suited to theparticular use contemplated. The features of the embodiments describedherein may be combined in all possible combinations of methods,apparatus, modules, systems, and computer program products.

1. A method, comprising: performing a wet chain processing on an inputaudio content, the wet chain processing comprising: producing upperharmonics associated with a first frequency band of the input audiocontent, the upper harmonics being located in a second frequency band;and filtering the upper harmonics using an infinite impulse responsebandpass filter to produce a wet chain audio component; performing a drychain processing on the input audio content, the dry chain processingcomprising: generating a dry chain group delay to match a wet chaingroup delay associated with the infinite impulse response bandpassfilter; and applying the dry chain group delay to the input audiocontent to the dry chain audio component; and combining the wet chainaudio component with the dry chain audio component.
 2. The method ofclaim 1, wherein the dry chain group delay is generated using at leastone all-pass filter to produce a group delay in the second band offrequencies that matches the group delay associated with the infiniteimpulse response bandpass filter.
 3. The method of claim 2, wherein eachall-pass filter is a second-order all-pass filter; and each all passfilter is configured to exhibit a particular group delay characteristicin a sub-band of frequencies within the second band of frequencies. 4.The method of claim 1, wherein the wet chain processing comprisesfiltering the input audio content to produce the first band offrequencies.
 5. The method of claim 4, wherein the first band offrequencies is produced using a first infinite impulse response filter.6. The method of claim 5, wherein the dry chain processing comprisesgenerating the dry chain group delay by matching a wet chain group delayassociated with the first infinite impulse response bandpass filter. 7.The method of claim 1, wherein the wet chain processing comprisesspectral shaping of the wet chain audio component.
 8. The method ofclaim 7, wherein the spectral shaping is carried out using a parametricfilter.
 9. The method of claim 8, wherein the parametric filter isconfigured to emphasize a lower sub-band of frequencies within thesecond band of frequencies.
 10. The method of claim 1, wherein the wetchain processing comprises generating and applying a delay to the wetchain audio component prior to the combining.
 11. The method of claim 1,wherein the wet chain processing comprises generating and applying again to the wet chain audio component prior to the combining.
 12. Themethod of claim 1, wherein the dry chain processing comprises generatingand applying a gain to the dry chain audio component prior to thecombining.
 13. The method of claim 1, wherein the input audio content isa single-channel audio content; and the dry chain processing and wetchain processing are carried out on the single-channel audio content.14. The method of claim 1, wherein the input audio content is amulti-channel audio content; and the dry chain processing and wet chainprocessing are carried out on individual channels of the multi-channelaudio content.
 15. The method of claim 1, wherein the input audiocontent is a multi-channel audio content; the dry chain processing iscarried out on individual channels of the multi-channel audio content;and at least a portion of the wet chain processing is carried out on acombined audio content that comprises two or more of the channels of themulti-channel audio content.
 16. A device, comprising: a processor; anda memory, comprising processor executable code, the processor executablecode, when executed by the processor, configures the device to: performa wet chain processing on an input audio content by configuring thedevice to: produce upper harmonics associated with a first frequencyband of the input audio content, the upper harmonics being located in asecond frequency band; and filter the upper harmonics using an infiniteimpulse response bandpass filter to produce a wet chain audio component;perform a dry chain processing on the input audio content by configuringthe device to: generate a dry chain group delay to match a wet chaingroup delay associated with the infinite impulse response bandpassfilter; and apply the dry chain group delay to the input audio contentto produce a dry chain audio component; and combine the wet chain audiocomponent with the dry chain audio component.
 17. The device of claim16, wherein the dry chain group delay is generated using at least oneall-pass filter configured to produce a group delay in the second bandof frequencies that matches the group delay associated with the infiniteimpulse response bandpass filter.
 18. The device of claim 17, whereineach all-pass filter is a second-order all-pass filter; and each allpass filter is configured to exhibit a particular group delaycharacteristic in a sub-band of frequencies within the second band offrequencies.
 19. The device of claim 16, wherein, as part of the wetchain processing, the processor executable code, when executed by theprocessor, configures the device to filter the input audio content toproduce the first band of frequencies.
 20. The device of claim 19,wherein the first band of frequencies is produced by a first infiniteimpulse response filter.
 21. The device of claim 20, wherein, as part ofthe dry chain processing, the processor executable code, when executedby the processor, configures the device to generate the dry chain groupdelay by matching a wet chain group delay associated with the firstinfinite impulse response bandpass filter.
 22. The device of claim 16,wherein, as part of the wet chain processing, the processor executablecode, when executed by the processor, configures the device tospectrally shape the wet chain audio component.
 23. The device of claim22, wherein a parametric filter is used to carry out the spectralshaping.
 24. The device of claim 23, wherein the parametric filter isconfigured to emphasize a lower sub-band of frequencies within thesecond band of frequencies.
 25. The device of claim 16, wherein, as partof the wet chain processing, the processor executable code, whenexecuted by the processor, configures the device to generate and apply adelay to the wet chain audio component prior to the combination with thedry chain audio component.
 26. The device of claim 16, wherein, as partof the wet chain processing, the processor executable code, whenexecuted by the processor, configures the device to generate and apply again to wet chain audio component prior to the combination with the drychain audio component.
 27. The device of claim 16, wherein, as part ofthe dry chain processing, the processor executable code, when executedby the processor, configures the device to generate and apply a gain todry chain audio component prior to the to the combination with the drychain audio component.
 28. The device of claim 16, wherein the inputaudio content is a single-channel audio content; and the processorexecutable code, when executed by the processor, configures the devicethe device to carry out the dry chain processing and wet chainprocessing on the single-channel audio content.
 29. The device of claim16, wherein the input audio content is a multi-channel audio content;and the processor executable code, when executed by the processor,configures the device the device to carry out the dry chain processingand the wet chain processing on individual channels of the multi-channelaudio content.
 30. The device of claim 16, wherein the input audiocontent is a multi-channel audio content; and the processor executablecode, when executed by the processor, configures the device the deviceto carry out the dry chain processing on individual channels of themulti-channel audio content, and at least a portion of the wet chainprocessing on a combined audio content that comprises two or more of thechannels of the multi-channel audio content.
 31. A computer programproduct, embodied on a non-transitory computer readable medium,comprising: computer code for performing a wet chain processing on aninput audio content, the wet chain processing comprising: producingupper harmonics associated with a first frequency band of the inputaudio content, the upper harmonics being located in a second frequencyband; and filtering the upper harmonics using an infinite impulseresponse bandpass filter to produce a wet chain audio component;computer code for performing a dry chain processing on the input audiocontent, the dry chain processing comprising: generating a dry chaingroup delay to match a wet chain group delay associated with theinfinite impulse response bandpass filter; and applying the dry chaingroup delay to the input audio content to produce a dry chain audiocomponent; and computer code for combining the wet chain audio componentwith the dry chain audio component.
 32. The computer program product ofclaim 31, wherein the dry chain group delay is generated using at leastone all-pass filter to produce a group delay in the second band offrequencies that matches the group delay associated with the infiniteimpulse response bandpass filter.
 33. The computer program product ofclaim 32, wherein each all-pass filter is a second-order all-passfilter; and each all pass filter is configured to exhibit a particulargroup delay characteristic in a sub-band of frequencies within thesecond band of frequencies.
 34. The computer program product of claim31, wherein the wet chain processing comprises filtering the input audiocontent to produce the first band of frequencies.
 35. The computerprogram product of claim 34, wherein the first band of frequencies isproduced using a first infinite impulse response filter.
 36. Thecomputer program product of claim 35, wherein the dry chain processingcomprises generating the dry chain group delay by matching a wet chaingroup delay associated with the first infinite impulse response bandpassfilter.